A brushless motor drive control circuit of this kind controls drive of a brushless motor by detecting a counter electromotive voltage at a 3-phase stator coil generated along with rotation of a rotor by means of a comparator and detecting a rotation position of the rotor based on the counter electromotive voltage without using a hole element (e.g. Japanese Patent Laying-Open No. 8-149879 (Patent Document 1)).
Here, because an amplitude of a counter electromotive voltage generated at the stator coil is proportional to a rotation speed of the rotor, at low-speed rotation, the amplitude of the counter electromotive voltage is smaller as compared with that obtained at high-speed rotation, so that its ratio to noise, that is, an S/N ratio is liable to be low. As a solution thereof, noise is removed through a low-pass filter before detection of a counter electromotive voltage by the comparator to assure a predetermined S/N ratio, thereby preventing degradation of counter electromotive voltage detection precision. More specifically, the noise is switching noise generated when the brushless motor is PWM-driven (pulse width modulation) or the like. In addition, at low-speed rotation includes at the start of rotor rotation and at the time of shift to rotation stop other than at stationary low-speed rotation.
Patent Document 1: Japanese Patent Laying-Open No. 8-149879